Excessive amounts of accumulated dispersion in high-speed optical communication systems can severely impair the quality of the transmitted signals. The effect of accumulated dispersion is particularly important in fiberoptic communication systems that transmit signals over long spans of standard single-mode fibers, which may exhibit chromatic dispersion of up to 17 ps/nm/km. Therefore, the chromatic dispersion in the fiber link is often compensated for using dispersion-compensating modules that are interspersed with the fiber spans and designed to substantially reduce the accumulated total chromatic dispersion in each fiber span. However, the exact amount of accumulated dispersion, which transmitted signals experience in the fiber spans and dispersion-compensating modules in a given link, often is unknown, because the fiber link was originally designed to transmit signals at substantially lower data rates, which are more tolerant to residual accumulated chromatic dispersion.
For this reason, it is often unclear whether a certain transmission system can be upgraded to transmit signals at higher data rates, which usually are more sensitive to residual accumulated chromatic dispersion. For example, various transmission systems that currently operate at data rates of 10 Gb/s are under consideration to be upgraded to data rates around 40 Gb/s.
Therefore, in order to assess whether a certain transmission link can be upgraded to transmit signals at higher data rates, the overall accumulated chromatic dispersion of said link has to be re-measured. The chromatic dispersion could be measured either individually for each fiber span and dispersion-compensating module or, alternatively, in a single end-to-end measurement. Obviously, a span-by-span measurement of the chromatic dispersion requires that the entire transmission link be temporarily taken out of service and that technical personnel be deployed to various remote locations to re-measure the accumulated dispersion in each individual span. Obviously, the costs associated with such an approach are extensive, if not prohibitive.
End-to-end measurements of the accumulated chromatic dispersion may be performed at considerably lower costs. Indeed, various measuring instruments are commercially available which allow measurements of the accumulated chromatic dispersion in the transmission link over a broad optical frequency range. For example, such instrument is manufactured by JDSU Corporation and known under the name ODM module for the series Tberd® 6000 or 8000 test and measurement system, which is capable of measuring accumulated dispersion in a long fiberoptic transmission link, in which the signal has to pass through various optical amplifiers and even optical filters. Unfortunately, the entire transmission link has to be taken out of service in order to perform such measurements, because the two ends of the transmission link have to be connected to the measurement equipment.
Unfortunately, such measurements may not be possible in certain modern optical networks that employ fixed or re-configurable optical add-drop multiplexers (OADMs), because the various optical signals launched into a given span may be sent to different geographic locations or, in some cases, over substantially different routes to the same location. In these modern networks, it becomes necessary to measure the accumulated chromatic dispersion separately for the specific routes over which the various optical signals or wavelength channels are transmitted.
Furthermore, the transmission of optical signals may be controlled in various nodes along the link, in such a way that these nodes pass the signals only when they exhibit certain optical powers and/or other predetermined qualities, such as optical bandwidth and carrier frequency. Therefore, the optical test signals transmitted over a given link by the chromatic dispersion measurement system have to comply with any given power and frequency requirements in order to be received at the other end of the link.
Chromatic dispersion in a transmission link can be measured using a variety of different measurement techniques. In conventional methods, the accumulated chromatic dispersion is estimated from periodically repeated measurements of the difference between the time of flight of two modulated optical signals having substantially different optical frequencies. A more detailed description of this measurement principle may be found, for example, in United States Patent Application Publication No. US2002/0186437 A1 “Chromatic-Dispersion Measuring Apparatus and Method” and U.S. Pat. No. 5,406,368 “Method and Apparatus for Chromatic Dispersion Measurements”.
Additional information on related or competing technologies can be found in U.S. Pat. No. 5,969,806, entitled “Chromatic Dispersion Measurement in a Fiber Optic Cable”; U.S. Pat. No. 4,752,125, entitled “Apparatus to Measure Fiber Dispersion”; U.S. Pat. No. 7,016,023, entitled “Chromatic Dispersion Measurement”; U.S. Pat. No. 6,734,958, entitled “Dispersion Measurement in Optical Networks”; and United States Patent Publication No. 2006/0109452, entitled “Device and Method for Chromatic Dispersion Measurement”.
Other monitoring methods are disclosed in U.S. Pat. No. 7,197,242, entitled “Chromatic-Dispersion Measuring Apparatus and Method”, issued to Sumitomo Electric; U.S. Pat. No. 6,912,359, entitled Methods for Monitoring Performance in Optical Networks”, issued to The Regents of the University of California; and U.S. Pat. No. 7,035,538, entitled “Monitoring Optical Dispersion Based on Vestigial Side Band Optical Filtering, issued to Willner et al of the University of Southern California
Willner et al. discloses a method for measuring chromatic dispersion in a conventional modulated optical information signal, whereas the present invention employs a specially modulated signal, which enables chromatic dispersion measurements over a substantially larger range and with substantially higher accuracy.
The method described by Willner et al. utilizes two modulated sidebands, but requires the presence of a vestigial optical carrier in the detected signal for measuring chromatic dispersion.
As described in the prior art, two optical test signals are generated by two separate optical laser sources CW Light Source 1 and CW Light Source 2, as illustrated schematically in FIG. 1. Subsequently, the two signals are multiplexed together in an Optical Multiplexer and simultaneously modulated with a sinusoidal signal using a common electro-optic amplitude modulator, e.g. Intensity Modulator. The optical carrier frequencies of the two signals are offset by a predetermined value, which has to be maintained precisely over the duration of the measurement. The two signals are then demultiplexed at the receiving end of the transmission fiber link, using a narrowband optical demultiplexor or two narrowband optical filters, and the optical power of the two signals is detected by two fast photo-detectors PD 1 and PD 2, which convert the intensity modulation in the optical signals into sinusoidally varying electrical currents. The difference between the time of flight of the two modulated test signals is then measured by an electrical phase meter that is connected to the two photo-detectors PD 1 and PD 2. If the fiber link does not exhibit any accumulated chromatic dispersion, the time of flight should be identical for both signals. In the presence of substantial uncompensated chromatic dispersion in the fiber link, the residual chromatic dispersion, D, can be calculated from the difference between the time of flight, Δt, and the frequency offset between the two optical signals, ΔF, asD=Δt/ΔF 
This method is often used to evaluate the frequency dependence chromatic dispersion over a broad optical frequency range, for which one may use a light source with fixed optical frequency to generate the first optical test signal and another light source with variable optical frequency source or, alternatively, a multitude of light sources at different, but fixed, optical frequencies to generate the second optical test signal.
Therefore, to measure the accumulated dispersion within a single optical wavelength channel usually would require two narrowband light sources, e.g. lasers, whose optical frequencies are spaced close enough to be transmitted through the clear optical bandwidth of the channel, which, depending on the particular transmission system, may be less than 50 GHz. Furthermore, to measure the accumulated dispersion within a different optical wavelength channel would require two narrowband light sources, e.g. lasers, at different optical frequencies or, alternatively, two frequency-tunable light sources that can be tuned to operate at a multitude of different optical frequencies. In addition, the narrowband optical demultiplexor at the receiving end of the fiber link would need to be tuned synchronously with the two light sources to detect the desired modulated signals. To those skilled in the arts, it is well known that precise and simultaneous tuning of two laser sources and two narrowband optical filters requires substantial efforts and, hence, would be prohibitively expensive.
Accordingly, an object of the present invention is to provide an apparatus that enables end-to-end measurements of accumulated dispersion in individual wavelength channels. Moreover, this apparatus transmits optical signals that are compatible with the optical bandwidths and power levels of conventional optical information signals transmitted over modern telecommunication systems. It is an aspect of this invention that these measurements may be performed in individual wavelength channels, which are temporarily taken out of service, and that they do not affect or otherwise involve the transmission of optical information signals in adjacent wavelength channels.
These highly desirable features are accomplished by designing the measurement apparatus in such a way that it transmits modulated optical test signals that can be generated with the same tunable light sources and optical modulators which are employed in standard telecommunication equipment. Furthermore, the transmitted test signals can be received by optical detectors that employ substantially similar optical components as used in commercial telecommunication signals.